This invention involves a two-step etch process that can be used to sequentially remove layers of material. This digital etching technique is particularly useful for the fabrication of semiconductor devices that require the ability to very accurately remove a few nanometers of material. One particular application is the conversion of AlGaN/GaN HFET/HEMT devices from depletion mode (normally-on operation) to enhancement mode (normally-off operation). Because of the polarization effects at the AlGaN—GaN interface, a 2DEG is formed at the interface [1], which creates a conduction path for normally-on, or depletion-mode (D-mode) devices, as shown in FIG. 1. FIG. 1 shows device 100 comprising a D-mode AlGaN/GaN HFET structure, with continuous 2DEG 126 between the source 150 and drain 170 contacts. The device 100 comprises a buffer layer 130, a GaN channel layer 120 and an AlGaN barrier layer 110. The device 100 in FIG. 1 further comprises a source 150, drain 170, gate 160 and passivation material 180, which may be SiN. The vertical energy band diagram of the device is also shown on the right, with the charge density plot below showing the concentration of the electrons 11 in the 2DEG 126 along the length of the 2DEG 126. The upper diagram on the right is the band energy diagram showing the conduction band 13, valence band 12 and Fermi level 14.
Depletion mode devices use a negatively biased gate on top of the AlGaN barrier layer to deplete the 2DEG of charge, and thus turn off the device. These devices are called “normally-on” because under no gate bias, there is a conduction path between the source and drain contacts. FIG. 1 shows the GaN channel layer of a depletion mode device, in contact with an approximately 200 angstrom AlGaN layer. The associated band diagram shows the Fermi energy level as function of depth of the AlGaN layer measured from the upper surface of the AlGaN layer and the corresponding charge density as a function of this depth. The charge density plot shows the presence of the 2DEG at the interface between the AlGaN and GaN layers. Note at this depth, the Conduction band dips below the Fermi energy level.
A “normally-off” device would have negligible conduction between the source and drain under no gate bias. A normally-off device would be useful for power applications for increased safety and lower power consumption. GaN enhancement mode (E-Mode) HFETs/HEMTs have been fabricated by (a) growing a thinner AlGaN barrier layer [2], (b) implanting fluorine into the barrier under the gate [3], and (c) etching the barrier layer under the gate [4]. The numbers in square brackets refer to the references that follow.
By growing a thinner AlGaN barrier layer 210, as shown in FIG. 2, the conduction band of the energy band diagram is shifted higher than the Fermi energy so that the 2DEG 226 does not exist, making the device normally-off However, the drawback to this approach is that the 2DEG 226 does not exist anywhere in the device, so there is no low-resistance, high-mobility conduction path between the source and drain. Furthermore, it is not practical to make a gate 260 long enough to span the entire source 250-drain 270 gap to modulate a channel 220.
By implanting fluorine ions into the AlGaN barrier under the gate, as shown in FIG. 3, the 2DEG is depleted under the gate near the fluorine ions, creating a break in the 2DEG channel between the source and drain. This break in the 2DEG channel results in a normally-off device, but because only the area under the gate is affected the gate is still able to modulate the channel by building up charge in that area and therefore turning the device on again. However this method suffers from a few drawbacks: (1) control of the fluorine dose is difficult, so repeatable fluorine treatment is an issue, (2) fluorine is usually implanted using a non-uniform low-power plasma hence the uniformity of the threshold voltage across a wafer is an issue when long gates are constructed, (3) the implant process inherently causes damage to the material, reducing performance, and (4) the stability of the implanted fluorine ions is unknown, so the reliability of the device is an issue.
The final method to create enhancement mode devices is to selectively remove the AlGaN barrier layer under the device gate, as shown in FIG. 4. This selective removal or “gate recess etch”, thins the AlGaN barrier sufficiently to shift the conduction band higher than the Fermi energy, as with the thin-grown AlGaN barrier method, depleting the 2DEG in the etched area under the gate. However, unlike the thin-grown AlGaN barrier method, the 2DEG is only depleted under the gate and continues to exist everywhere else in the device. Therefore, the device is normally-off, but the gate is still able to modulate the channel. Gate recess etching is usually achieved with a plasma dry etching system, such as RIE (Reactive Ion Etching) or ICP (Inductively Coupled Plasma). However, problems with the traditional method of gate recess etching include (1) that the plasma is usually not very uniform resulting in non-uniform threshold voltages as with fluorine treatment, and (2) the etches are usually done at sufficiently high plasma powers to cause deep damage in the material that is not removed by the etching process.